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Gatwick Airport in London warned that some flights may be delayed throughout Saturday and apologized for the inconvenience. Flights were delayed by up to three hours late Friday because of poor weather conditions, and at least 40 flights from the airport, mostly short haul to Europe, were delayed Saturday morning.

Heathrow Airport, one of Europe’s busiest, also advised passengers to check with their airlines regarding potential delays.

The U.K.’s weather forecasters, the Met Office, said thick fog patches could reduce visibility to just 100 meters (328 feet) in some areas across the country. It said travelers should allow “a little bit longer” for journeys and warned drivers to be extra careful.

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Conditions are expected to improve Sunday, the Met Office said.

The country’s main air traffic control organization said the restrictions would remain in place in areas with low visibility.

“Restrictions of this sort are only ever applied to maintain safety,” it said. “Our teams are working closely with the airports and airlines to minimise disruption.”

The restrictions came at a busy time of year when many people travel following the Christmas holiday and ahead of the New Year.

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